It is desirable to be able to perform energy metering at the point of delivery of natural gas, at least for certain users, such as industry, or for certain groups of users within a public distribution network.
Energy metering at the point of delivery is defined as measuring, in a single location, magnitudes that enable the quantity of energy delivered to be determined. These magnitudes are, in particular, volume, pressure, temperature, gas composition, calorific value (CV), energy flow rate.
The point of delivery corresponds to the expansion station where the gas is expanded, regulated, and metered. Such a station is designed to guarantee continuity of gas supply even in the event of one of the elements of the station failing.
The composition of natural gas as supplied to transport and distribution networks can vary, which gives rise to variations in the calorific value of the gas, and consequently to variations in the mount of energy supplied for given volume of delivered gas.
Industrial clients of transport and distribution networks desire to be able to optimize methods that require the use of gas, and consequently, even if they do not know the exact composition of the natural gas used, they would still like to be able to determine the calorific value of the natural gas they are consuming.
For their part, the suppliers of gas would like to know the calorific value of the gas they have delivered so as to be able to bill their clients as a function of the mount of energy actually supplied instead of merely as a function of the volume of gas delivered, since volume corresponds to varying mounts of energy depending on the calorific value of the gas.
For all of these reasons, it is desirable to be able to determine the calorific value (specifically the calorific value or CV) of a natural gas with accuracy to within better than 1%, and if possible to within about 0.5%, by means of a method that is of reasonable cost and that is easy to implement.
Proposals have already been made to measure CV using fast chromatographs or calorimeters. Those methods remain relatively expensive and they are not fully satisfactory for the intended application since they require samples of the gas to be taken, measurements to be performed in special test cabins, and standard gas to be used, all of which implies that the length of time required to perform a measurement prevents it from being done in real time.
Proposals have also been made, in particular in document FR-A-2 735 236, to use an infrared technique to measure the calorific value of a natural gas. In that technique, a specific absorption line in the near infrared is initially selected for each component of the gas, the gas is illuminated by a laser beam having a wavelength close to the wavelength of the selected absorption lines, and a spectrum width that is narrower than the widths of the selected absorption lines, the wavelength of the light beam is varied to scan through the selected absorption lines, the intensity of the light beam after passing through the gas is measured both at the wavelengths of the selected absorption lines and at other wavelengths, and the relative concentrations of the components of the gas are deduced therefrom by calculation and hence the calorific value of the gas can also be deduced.
Such a method requires the use of a system having a tunable laser diode or the equivalent, which is expensive, it makes use of high resolution spectroscopic techniques which remain complex to implement, it requires good prior knowledge about the nature and the composition of the gas in order subsequently to be able to determine its calorific value with sufficient accuracy, and it cannot be applied appropriately to natural gases whose components do not present a structure with well-isolated absorption lines, or that present absorption lines that overlap completely or in part.